essential amino acid An amino acid that an organism is unable to synthesize in sufficient quantities. It must therefore be present in the diet. In humans the essential amino acids are arginine, histidine, lysine, threonine, methionine, isoleucine, leucine, valine, phenylalanine, and tryptophan. These are required for protein synthesis and deficiency leads to retarded growth and other symptoms. Most of the amino acids required by humans are also essential for all other multicellular animals and for most protozoans.

essential amino acid n. an amino acid that is essential for normal growth and development but cannot be synthesized by the body and must therefore be obtained from protein in the diet. See amino acid.